In the recent years, an ATM-PON (Asynchronous Transfer Mode-Passive Optical Network) system has been employed for efficient communications using an ATM (Asynchronous Transfer Mode) network. FIG. 21 is an illustration of a configuration of the ATM-PON system. An ATM-PON system shown in FIG. 21 is of a type, for example, providing a telephone service or video service, and is made up of an ONU (Optical Network Unit) group 70, an ODN (Optical Distribution Network) 71, an ATM-optical subscriber line terminating unit 75, an SDH trunk-system multiplexer 76, a router 74, a DCN (Digital Connection Network) 73 and an OPS (Operation System) 72.
In this configuration, the ONU group 70 includes a plurality of ONUs, for example, 64 ONUs #1 to #64 as shown in FIG. 21. These ONUs are connected to homes to accommodate the ordinary users. Moreover, each of the ONUs interchanges serial numbers or identification information with the ODN 71 to allow the ODN 71 to identify terminal names. A sequence therefor will be described later with reference to FIG. 27.
In addition, the ODN 71 is for time-division-multiplexing ATM-PON cells in a direction from the ONU group 70 to the ATM-optical subscriber line terminating unit 75 and for branching ATM-PON cells in a direction from the ATM-optical subscriber line terminating unit 75 to the ONU group 70.
The ATM-optical subscriber line terminating unit 75 is for establishing an interface between optical subscribers on the ONU group 70 side and an ATM network on the SDH trunk-system (backbone) multiplexer 76 side, and further for controlling transmissions.
FIG. 22(a) is an illustration of a configuration of a 150-Mbps down stream frame, and FIG. 22(b) is an illustration of a configuration of a 150-Mbps up stream frame. The down stream frame shown in FIG. 22(a) is a frame to be transmitted in a direction from the ATM-optical subscriber line terminating unit 75 to the ODN 71, while the up stream frame shown in FIG. 22(b) is a frame to be transmitted in a direction from the ODN 71 to the ATM-optical subscriber line terminating unit 75.
In FIG. 22(a), PLOAMs #1 and #2 included in the down stream frame are performance monitoring cells, while ATM Cell #1 to ATM Cell #54 included therein are user cells. These performance monitoring cells are cells for monitoring transmission lines, and are automatically sent whenever the transmission side transmits 27 user cells.
Concretely, The PLOAM #1 includes information on a path connection-accepting side, or others. An ATM header indicated by circled numeral 1 is a header section of an ATM cell transmitted, an IDEN indicated by circled numeral 2 is for identifying the head of a PLOAM cell, and a SYNC indicated by circled numeral 3 holds a count value written for timing generation. Moreover, GRANT 1 to 27 indicated by circled numeral 4 contains transmission grant information on an up-transmission line with respect to each ONU. The grant contents are seven in number. Still moreover, a message•PON-ID indicated by circled numeral 5 signifies a destination ID on the message transmitting side, a message•ID indicated by circled numeral 6 signifies a message•type identifier, and a message• field indicated by circled numeral 7 is for displaying message contents. Yet moreover, a BIP indicated by circled numeral 8 signifies a code for calculation of a bit error rate of ATM cells before the PLOAM cell.
Furthermore, the up stream frame shown in FIG. 22(b) is a time-division-multiplexed frame comprising user cells ATM Cell #1 to ATM Cell #53. Bits (screened) additionally placed before each of the ATM Cells represent three-byte overhead, which is for controlling the contents in accordance with a message in a down-PLOAM cell.
In addition, in FIG. 21, the SDH trunk-system multiplexer 76 is for transmitting main signal data through the use of ATM cells, the router 74 serves as a station at transmission of the main signal data, and the DCN 73 denotes a digital network. Still additionally, the OPS 72 is for remotely implementing centralized control of the ONU group 70 or the ATM-optical subscriber line terminating unit 75.
Still furthermore, in FIG. 21, coarse dotted lines depict control paths (control connections) between the ATM-optical subscriber line terminating unit 75 and the ONU group 70, while a fine dotted line depicts a main signal data path (main signal connection) between the ATM-optical subscriber line terminating unit 75 and the ONU group 70.
These paths establish multi-connection. For example, main signal data from the ONU group 70 existing in a city A pass through the main signal data path and are time-division-multiplexed in the ODN 71, and then are transmitted through the ATM-optical subscriber line terminating unit 75 to the SDH trunk-system multiplexer 76 existing in a city B. In addition, control signals from the ONU group 70 pass through the control paths and are time-division-multiplexed in the ODN 71 and then inputted to the ATM-optical subscriber line terminating unit 75, but not transmitted from this ATM-optical subscriber line terminating unit 75 to the external.
Moreover, in the ATM-optical subscriber line terminating unit 75 shown in FIG. 21, a PON-IF section 75d on the ODN 71 side is for performing format conversion between an ATM cell and an ATM-PON cell, an ATM-SW 75c is for switching main signal data, and an ATM-IF section 75e for making an interface for cell transmission between the ATM-optical subscriber line terminating unit 75 and the SDH trunk-system multiplexer 76.
Still moreover, a SEMF-IF section 75a connected to the router 74 side is for extracting a control command from a signal transmitted from an OPS 72 to send it to an OAM-IF section 80, and concretely, is for performing format conversion of a frame from the OPS 72.
Yet moreover, the OAM-IF section 80 is for receiving and assembling only one path for ATM cells inputted from the PON-IF section 75d to perform conversion to an adaptation layer AAL5 (ATM Adaptation Layer 5) message. Concretely, this OAM-IF section 80 assembles only one of a plurality of cells coming from the ONU group 70 through the use of a receiving buffer on the basis of a control command from the SEMF-IF section 75a and sends it out as an AAL5 message to the ATM-IF section 75e. 
This AAL5 message signifies one of AAL0, AAL1, AAL3/4, AAL4 and AAL5 of an adaptation layer, different according to data type or data rate. As well known, the layer structure in the ATM communication comprises a physical layer, an ATM layer forming an upper layer of the physical layer for establishing connections in an ATM network to interchange ATM cells, and an ATM adaptation layer forming an upper layer of the ATM layer for translating information data and ATM cells. For example, with respect to a large volume of information data, ATM cells divided in units of 48 bytes are generated in the ATM adaptation layer and processed by the ATM layer and the physical layer. Moreover, this adaptation layer is classified into AAL0, AAL1, AAL3/4, AAL4 and AAL5 on the basis of information data types or information data rates.
Of these, the AAL5 is a layer for transmitting IP data or the like through an ATM network. In this AAL5, a variable-length pad (pseudo-data) is added to transmission data to form a multiple of 48 bytes and the pad-added data is CRC-operation-processed so that a parity is added to the variable-length pad added data. All the data thus obtained are outputted as CS-PDU which in turn, produces ATM cells divided into segments of 48 bytes, which can prevent bit errors or cell loss.
Thus, for example, ATM-PON cells passing through a plurality of paths from 64 ONUs #1 to #64 are time-division-multiplexed in the ODN 71 and converted into ATM cells in the PON-IF section 75d and further switching-processed in the ATM-SW 75c. These ATM cells are sent from the ATM-IF section 75e to the SDH trunk-system multiplexer 76. Moreover, the ATM cells sent from the SDH trunk-system multiplexer 76 are inputted through the ATM-IF section 75e to the OAM-IF section 80.
At this time, in this OAM-IF section 80, the receive assembling is made with respect to only one of a plurality of paths, thus conducting conversion into an AAL5 message. In addition, this AAL5 message is sent out from the SDH trunk-system multiplexer 76. In this connection, this path information indicates a path identified by the VPi (Virtual Path identifier) and VCi (Virtual Channel identifier) in an ATM cell header.
FIG. 23 is a block diagram showing a conventional OAM-IF section. In FIG. 23, an OAM-IF section 80 is composed of an ATM cell assembling unit 81 and an external memory (receiving buffer)82. In this configuration, a path recognizing section 81a is for extracting path information from a received cell to abandon the received cell other than the path information on the receive assembling processing. A received message assembling section 81b is for conducting cell assembling on an AAL5 message having the path information on the assembling to output readout information, and an external memory control 81d is for controlling the readout from the external memory 82.
In this connection, in FIG. 21, in a case in which a timing at which the OAM-IF section 80 receives a command from the SEMF-IF section 75a and a timing at which message assembling processing is conducted in the external memory 82 differ time-wise from each other, an arbitrating section 81c mediates this time-wise difference, with it being controlled by a signal inputted from a CPU control section (not shown).
Secondly, a method of selecting one of received cells in the above-described configuration will be described herein below with reference to FIG. 24. FIG. 24 is a flowchart showing conventional received cell assembling processing. As FIG. 24 shows, upon reception of an ATM cell (step F1), a step F2 is implemented to extract path information from the received cell for checking whether or not it agrees with path information on receive assembling processing. If the agreement takes place, the YES route is taken for conducting cell assembling with respect to an AAL5 message having the assembling path information, and data on the received cell is written in the external memory 82 (step F3). On the other hand, if the checking result in the step F2 indicates no agreement, the NO route is taken for abandon the received cell other than that path information (step F4).
FIG. 25 is an illustrative view showing a flow from the ATM cell reception to the AAL5 message assembling. In FIG. 25, the received message assembling section 81b conducts the AAL5 message assembling with respect to an ATM cell transmitted through a path 1. Incidentally, this path information (for example, information indicative of path 1) is set on both transmission and reception.
FIG. 26 is an illustrative view showing a flow to be taken for when, upon the reception of an ATM cell, the received cell is abandoned. In FIG. 26, the received message assembling section 81b conducts the AAL5 message assembling with respect to the path 1. Moreover, when ATM cells 82a, 82b and 82c transmitted through the path 1 and ATM cells 83a and 83b transmitted through a path 2 are inputted to this received message assembling section 81b, the ATM cells 82a, 82b and 82c from the path 1 undergo the AAL5 message assembling in the external memory 82. On the other hand, the ATM cells 83a and 83b from the path 2 other than the set path, which have come in, are not subjected to the AAL5 message assembling on the path 2, but are abandoned.
A description will be added of the above-mentioned ONU. FIG. 27 is an illustration of a start-up sequence of the ONU. As FIG. 27 shows, in response to the power-on, the ONU takes an initial state (step P1), and upon the establishment the synchronization with the ODN 71, it enters a delay measurement standby state −1 (step P2). Then, after the setting of parameters, it falls into a delay measurement standby state −2 (step P3). When serial numbers are interchanged with respect to the PON-IF section 75d and the serial numbers are identical to each other, the YES route is taken so that it becomes a delay measurement standby state −3 (step P4).
Following this, examination is made on an optical laser output. If the examination shows a satisfactory result, the YES route is taken so that it falls in an operation standby state −1 (step P5). At this time, the serial numbers are again interchanged with respect to the PON-IF section 75d and an identification number is communicated thereto, and it enters an operation standby state −2 (step P6). Moreover, identification numbers for identifying a plurality of ONUs are added to the PON-IF section 75d and it enters an operation standby state −3 (step P7). In response to a notice on a delay adjustment indication value, the YES route is taken so that it takes an operating state (step P8). Furthermore, the setting of a path takes place (step P9) to perform interchange of ATM cells (step P10). In the steps P3 to P8, if there is no reception of a signal for state transition, the NO route takes place so that the flow returns to the step P2 processing.
As described above, this ATM-PON system 90 supports only a function to assemble only the AAL5 message having path information previously set for both transmission and reception. Accordingly, when an AAL5 message is received through a path other than the set path, there is no choice other than to abandon it. This creates a problem in that difficulty is encountered in achieving large volume transmission/reception. In addition, in assembling an AAL5 message from the path 2, there is a need to switch the path setting, and if a plurality of paths exist, there is a need to alter the setting on each of the paths so that transfer processing takes much time.
The present invention has been developed with a view to eliminating such problems, and it is therefore an object of the invention to provide a message writing apparatus, message writing method, message readout apparatus, message readout method, memory address control circuit for writing of a variable-length message and memory address control circuit for readout of a variable-length message, operable of, in the case of multi-connections, processing AAL5 messages from a plurality of paths by, when received ATM cells are read/written from/in a receiving buffer, reading/writing in a memory area corresponding to each path and further of shortening a data transfer time by improving the transfer processing capability.